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Performance and Limitations

The Performance section of your POH is a set of predictions. The Limitations section is a set of walls. Most of learning this subject is knowing which one you are looking at, and how much margin you have already spent before you release the brakes.

Technical Subject Areas — FI.II.F

The question the numbers answer

Halfway down the takeoff roll is the worst place to learn that this airplane will not get off this runway today, and a few seconds later that it will not clear the obstacle at the departure end either. Every chart in this subject exists so you find that out on the ground with a pencil, while the answer still costs you nothing but a delay.

14 CFR 91.103 begins: "Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight." Paragraph (b) then requires, in its own words, "For any flight," runway lengths at airports of intended use together with the takeoff and landing distance data contained in the approved flight manual. Note the phrase: any flight. Not the tight ones, not the short ones, not the ones where the runway looks like it might be a problem — any flight. 14 CFR 91.9(a) then prohibits operating a civil aircraft without complying with the operating limitations specified in the approved flight manual, markings, and placards. The placards are the half you actually read in the cockpit. Go to the sections themselves and read them whole; the Code of Federal Regulations is the authority, it changes, and this page is not it.

The two words are not interchangeable. Performance is how well the airplane takes off, climbs, cruises, and lands, computed for a particular weight, altitude, and temperature. Limitations are the boundaries of safe operation: maximum weights, the CG range, speed and load-factor restrictions, and the markings and placards that repeat them where you can see them. Miss a performance number and you are behind the airplane. Cross a limitation and you are outside the envelope the airplane was tested and certificated in, where nobody has published what happens next.

Weight and balance is where it starts

Airplanes are sensitive to how much you load and where you put it. The vocabulary is small. Maximum gross weight is the most the airplane may weigh. Basic empty weight is the standard empty airframe plus installed optional equipment, including unusable fuel and operating fluids. Useful load is maximum allowable gross weight minus basic empty weight — everything you are allowed to add. Payload is occupants, cargo, and baggage.

Balance uses a second set. The datum is an imaginary vertical plane from which all horizontal distances are measured, and the manufacturer picks where it sits. An arm is the distance from datum to item. A moment is the turning effect that item has about the datum — a torque, weight multiplied by arm, not a force in its own right. Total the weights, total the moments, divide moments by weight, and you have the center of gravity. Both answers have to pass: a load can be under gross weight and still out of limits because it sits too far aft.

Where the CG sits, and what it costs you

On a well-designed airplane the CG is ahead of the center of lift, so the airplane wants to pitch nose down and the tail carries a download to hold the nose up. Moving the CG inside the envelope changes how strongly that works.

A forward CG buys stability and pays in performance. The tail works harder, so there is more drag and slightly less cruise speed, and the wing carries more than the airplane's actual weight, so stall speed goes up. Stall recovery is better. Push to the forward limit and raising the nose gets heavy — on rotation, and again in the flare, where you can run out of elevator before you run out of runway.

An aft CG is the mirror. Less tail download means less drag, a little more cruise speed, and a lower stall speed, which is why it tempts people. What you give up is longitudinal stability: control forces go light, making the airplane easy to overstress, and stall and spin recovery gets worse. Performance improves right up until controllability disappears.

A crew once loaded baggage without securing it. The airplane was within limits on the ramp; after takeoff the load slid aft, the CG went with it, and the airplane they were flying was not the one they had computed. Tying the load down is a weight and balance action, not a tidiness one — a CG that moves once you are airborne moves at the moment you have the fewest options.

Adding, removing, and shifting weight

Weight changes constantly in flight as fuel burns, at roughly six pounds per gallon of avgas. In most light airplanes the tanks sit near the CG, so burning fuel changes what the airplane weighs far more than where it balances — verify that in your POH rather than assuming it across types.

For a load you move on the ground, the shift relationship is the one to know: weight shifted divided by total weight equals change in CG divided by the distance the weight moved. To add or remove weight instead, work it the long way — add or subtract the item's weight and its moment, then divide the new total moment by the new total weight. Changes to fixed equipment move both numbers, and maintenance must produce a revised record when they do; fly from the current record for the airplane you are actually in.

What extra weight does

An overloaded airplane does not simply feel sluggish. Everything moves the wrong way:

• Higher takeoff speed and a longer takeoff roll
• Reduced rate and angle of climb, and a lower maximum altitude
• Reduced cruise speed and shorter range
• Reduced maneuverability and a higher stall speed
• Higher approach speed and a longer landing roll

None of that is a switch that flips at maximum gross weight — it is a gradient. Overloading is the illegal end of it, but a legally loaded airplane at gross on a hot day is already living most of that list. The question before departure is not only whether you are legal, but how much margin is left.

The air you are asking it to fly in

Air density is set by pressure, temperature, and humidity, and falls as you climb. As air thins the engine makes less power, the propeller less thrust, and the wing less lift — three losses stacking at once. Pressure altitude is what the altimeter reads set to 29.92 inches. Without changing the setting, you can compute it: subtract the current altimeter setting from 29.92, multiply by 1,000, and add that to field elevation. The order matters. It is 29.92 minus the setting, so that low pressure gives you a positive correction and a pressure altitude above field elevation; reverse the subtraction and the answer comes out several hundred feet optimistic on exactly the day you could least afford it. Density altitude is pressure altitude corrected for nonstandard temperature, roughly 120 feet for every degree Celsius the air sits above standard. It is the altitude the airplane thinks it is at, and it decides whether the chart you are reading has anything to do with today. The temperature and altimeter setting you need are sitting in the METAR. Run both: pressure altitude and density altitude.

The airport, the airplane, and you

Charted numbers assume a paved, level, dry surface. Grass, gravel, or soft ground adds rolling resistance and lengthens the roll, and your POH may not offer a correction for it. A downhill slope helps the takeoff and hurts the landing. Terrain matters as much as the field. Telluride sits on a mesa at better than nine thousand feet, its runway ends dropping away into the canyon and the San Juans standing around the valley, so a summer afternoon there stacks high elevation and heat against an airport that gives you no overrun at either end and very little room to maneuver low. A takeoff roll that fits the pavement is not the same as a climb the surrounding country will accept.

Technique and configuration are yours: flap setting, leaning for the conditions, brake release with power set, holding the correct speed. The chart assumes you do all of it correctly, which points at the honest limit of the subject. Calculated performance is not actual performance. Those numbers came from a new airframe and a new engine flown by a test pilot on a good day. Treat the chart answer as a floor, and decide before you take the runway what distance remaining means you abort.

Speeds, and reading them off a chart

The V-speeds in your POH are not constants. Rotation speed, best angle and best rate of climb, both stall speeds, best glide, and maneuvering speed are published for a stated weight, and several change as the airplane gets lighter. Maneuvering speed is the clearest case: it decreases at lower weights. A lighter airplane stalls at a lower airspeed, and Va is the speed at which the wing will stall — and stop carrying load — before the limit load factor is exceeded. Take weight out and that protective stall arrives sooner, so the speed that guarantees it comes down with the weight. Fly yesterday's Va at today's lighter weight and the wing can reach limit load before it lets go.

Va is also narrower protection than the name suggests. It is figured for full deflection of a single control. It does not account for several controls deflected at once, or for a control taken to full deflection and then reversed, either of which can overstress the airframe below Va. Get the number right and then keep the inputs honest.

Charts rarely list your exact weight, altitude, and temperature, so interpolate between the bracketing values rather than rounding to whichever neighbor you prefer — and read the notes in the margin, because that is where the chart's assumptions hide.

Where you meet it next

On a checkride this is oral-room material first. The examiner hands you a loading problem and a set of conditions, expects a defensible answer, then asks what you would do differently at the aft limit. In the airplane it shows up as the flare that needed more elevator than usual, the climb that would not come, and the July afternoon when the runway that was generous in March is not.

It reappears wherever a number must be met: slow flight and stalls, where CG position is felt directly; short-field and soft-field takeoffs, which are these charts flown; weather information, where the density altitude ingredients come from; and cross-country planning, where fuel and payload trade against each other on one useful load. Each of those has its own lesson in the ground school.